Yes, the checksum is useless, because the checksummed file is corrupt. By contrast, without checksumming... the file would still be corrupt! In the best case,

Yes, the checksum is useless, because the checksummed file is corrupt. By contrast, without checksumming... the file would still be corrupt! In the best case, when good data was written to disk, checksumming preserves the good data. In the worst case, when bad data was written to disk, checksumming does no harm. Checksumming is good.